The present invention relates to an image forming apparatus and moisture feeding device having a moisture feeding unit for giving moisture to a sheet onto which an image is particularly fixed by a heat fixing unit.
In general, a toner image is fixed onto a sheet by forming an image onto the sheet by using fine powder toner, heating and pressing the toner image onto the sheet in an image forming apparatus for forming an image by an electrophotographic method.
Due to the heating process in a fixing unit, moisture evaporates from the sheet. However, since the sheet is exposed to the environment, the sheet absorbs the moisture. Consequently the moisture included in the sheet is gradually recovered. However, the recovery of the moisture is not uniform. The recovery of the moisture proceeds in different rates depending on the portion of the sheet. For example, when the sheets are placed in a condition that a plurality of sheets are bundled, the process of absorbing moisture in a peripheral portion of the sheet proceeds in a high rate, however the process of absorbing moisture in a center portion of the sheet proceeds in a low rate or hardly proceeds.
Accordingly, since the elongation percentage of the sheet is different depending on the portion of the sheet due to the differences of the moisture content, the sheet tents to be shaped in waves and curl.
Even though this phenomenon varies depending on the kind of sheet, this phenomenon tends to occur in a place where a plurality of sheets is placed. This phenomenon tends to occur with sheets placed in the ejection tray where sheets onto which an image has been formed are ejected and with sheets placed in a stacker for a binding process, particularly in an ejection tray configured by a stacker having large capacity.
Since due to the differences of moisture contents, the elongation percentage of the sheet is different depending on the portion of the sheet, a waving phenomenon occurs.
There is a case that a curl phenomenon occurs with a sheet due to that the sheet curves.
U.S. Pat. No. 5,264,899 (Patent Document 1) discloses a device for adding moisture to a sheet to prevent development of edge waves formed along with the sheet by adding water from a shaft center of a pair of humidifying rollers by using a pair of porous humidifying rollers for conveying sheets.